This invention relates to a semi-tension mask, also called a semi-stretched-tension (SST) mask which is formed of a Fexe2x80x94Ni based alloy and is for use in a cathode ray tube (also called Braun tube). More particularly, this invention relates to such a Fexe2x80x94Ni based alloy which is low in thermal expansion and yet has excellent creep properties, and which upon baking after subjection to tension for mask formation, is capable of suppressing mask wrinkling. The invention also relates to a semi-tension mask made of the alloy material, and further to a color picture tube using the semi-tension mask.
Masks for picture tubes are roughly divided into two types; (1) the shadow mask type in which a mask material is formed with electron-beam passing dots or slots by etching and then is pressformed to a mask form, and (2) the aperture grille type in which a mask material is formed with electron-beam passing vertical slits by etching and then is stretched, in two upward and downward directions and mounted onto a frame.
For the shadow mask type, Fexe2x80x9436%Ni alloy (called xe2x80x9cinvarxe2x80x9d alloy) is commonly used since this alloy has a very low coefficient of thermal expansion and can control the doming (expanding in a dome form) phenomenon that results from thermal expansion. For the aperture grille type whose structural features rarely generates doming phenomenon due to thermal expansion, soft steel that is higher in the coefficient of thermal expansion but is less costly is employed.
These two mask types have both merits and demerits and have been used respectively in the market. More recently, what is known as a semi-tension mask type has come on the stage as a new type combining the merits of the shadow mask and aperture grille mask types.
The semi-tension mask is made by providing mask material formed with electron-passing dots or slots by etching. The etched mask material is then supported on a frame while stretching (tensioning) in two upward and downward directions, as in the aperture grille type, rather than press-forming. At the beginning of development of this new type mask, the mask was stretched in four directions, namely leftward and rightward directions as well as upward and downward directions with a relatively strong force. But, when stretched in four directions with a relatively strong force, the mask often broke. In order to avoid such possible breakage of the mask, an improvement was tried, namely, to stretch the mask only in two upward and downward directions with a relatively weak force, attaining satisfactory results. A mask made by this improvement came to be called the xe2x80x9csemi-stretched tension maskxe2x80x9d, or xe2x80x9csemi-tension maskxe2x80x9d for short in the sense that stretching with a relatively weak force in two directions only was adopted.
FIGS. 1(a) and (b) is an explanatory view schematically illustrating the semi-tension type mask and the aperture grille type mask, respectively. Both types of masks are stretched in upward and downward directions. In the semi-tension type mask, a number of vertical slot rows are formed over the mask breadth. Each slot row is composed of a number of slots with bridges left between adjacent slots. The aperture grille type mask includes a number of vertical long slits over the mask breadth. This necessitates damper wires for suppressing mask vibration caused by sound sources, such as speakers. The bridges in the semi-tension type mask are metal portions left unetched between the slots in each vertical slot row during etching. The bridges function to prevent torsion of the vertical slot rows. The semi-tension mask is also called the xe2x80x9cbridged tension maskxe2x80x9d due to the bridges present in each slot row.
Compared with the shadow mask type that depends on press forming, the new semi-tension type permits a flatter picture tube with greater brightness and higher resolution. Moreover, the semi-tension mask type is superior to the aperture grilled type in oscillation characteristics due to the presence of the bridges. Damper wires are not necessary. The semi-tension mask only requires relatively low loads for vertical stretching, making for cost reduction.
On the other hand, the semi-tension mask type is subject to doming upon thermal expansion, unlike the aperture grille type. To prevent this phenomenon, the use of Fexe2x80x94Ni based alloys of low thermal expansion, centered around the invar allow, is under study. It has, however, been found that the use of conventional Fexe2x80x94Ni based alloys including the invar alloy causes xe2x80x9ctension downxe2x80x9d or relaxation of tension in the mask upon heat treatment during the course of assembling, leading to major troubles such as wrinkling of the mask.
Thus, ordinary Fexe2x80x94Ni alloys such as the invar alloy have not been found appropriate for semi-tension masks. Detailed investigation into the individual steps of mask manufacture has revealed that the inappropriateness of Fexe2x80x94Ni alloys is attributable to the creep properties of the material.
In the course of manufacture, a mask material formed with dots or slots by etching is subjected to blackening treatment. Blackening treatment herein refers to a treatment for forming a black-colored film such as a film of iron oxide, on the surface of a mask material. The blackening treated mask material is welded to a frame and stretched under a predetermined load, and thereafter baked to eliminate strains that have resulted from welding and other operations. At the time of this baking, it has recently been found that the conventional Fexe2x80x94Ni alloy material under frame tension undergoes plastic deformation at elevated temperature, or creeping. Once it occurs, creeping causes elongation of the mask accompanied with xe2x80x9ctension downxe2x80x9d or load relief. This in turn leads to mask wrinkling, deterioration of anti-oscillation characteristics, and various other problems. Although solid solution strengthening through addition of Cu, Nb, Mo, W, and Ta has been found useful in decreasing creep elongation, the addition of such elements inevitably increases the coefficient of thermal expansion of the resulting alloy.
It has now been found that the addition of B, control of the N content, controlled addition of Al and Si, control of impurities C, P and S and heat treatment during the course of working, with strain relief annealing as desired, will markedly ameliorate the creep properties of the resulting alloy without adversely affecting its coefficient of thermal expansion, with no possibility of wrinkling or other defect in the steps of blackening-treating, stretching, and baking.
Based on this finding, the present invention solves the above problems by providing:
(1) a low thermal expansion Fexe2x80x94Ni based alloy for semi-tension masks with excellent creep properties characterized by consisting of from 34 to 38% Ni, from 0.01 to 0.5% Mn, from 0.0003 to 0.0015% B, from 0.0010 to 0.0050% N, and the balance Fe and unavoidable impurities,
(2) the Fexe2x80x94Ni based alloy according to (1) which further includes from 0.005 to 0.20% Si and from 0.005 to 0.030% Al,
(3) the Fexe2x80x94Ni based alloy according to (1) or (2) wherein, among the unavoidable impurities, C is controlled to no more than 0.010%, P is controlled to no more than 0.015%, and S is controlled to no more than 0.010%,
(4) the Fexe2x80x94Ni based alloy according to (1), (2) or (3) which is subjected, at least once after hot rolling or cold rolling, to annealing in a non-oxidizing atmosphere at between 650xc2x0 C. and 750xc2x0 C. for from 30 minutes to less than 5 hours, and
(5) the Fexe2x80x94Ni based alloy according to (4) which is subjected to stress relief annealing after final cold rolling.
This invention further provides:
(6) a semi-tension mask using an Fexe2x80x94Ni based alloy according to any of (1) to (5), wherein the mask member of said Fexe2x80x94Ni based alloy is formed with dots or slots by etching followed by blackening treatment, and then is supported on a frame while stretching in upward and downward directions, and
(7) a color picture tube using the semi-tension mask according to (6).